1. Field of the Invention
The present invention relates to an inductor whose energy loss is small.
2. Description of the Background Art
A small and high-density integrated monolithic microwave integrated circuit (MMIC) has attracted attention as a high-frequency circuit used for mobile communication, satellite communication or the like. This is a microwave integrated circuit of a multi-layer structure in which an active device (a field effect transistor (FET), a high electron mobility transistor (HEMT) or the like) and a passive device (a transmission line, a capacitor, an inductor or the like) are mounted together on a semiconductor substrate. In recent years, the high-frequency circuit has been fabricated with complementary metal-oxide semiconductor (CMOS) circuitry. In this MMIC, a spiral inductor is often used as an inductor. The spiral inductor has a spiral, electrically conductive pattern wound in a plane. At least one of the outside and inside terminals of the conductive pattern is electrically connected to an electrode on another wiring layer through a via hole or a through hole. The spiral inductor has an advantage that the number of wiring layers necessary for forming the inductor is reduced.
When a voltage is applied between the outside and inside terminals of the spiral inductor, a spiral current flows in the conductive pattern to induce magnetic field around the inductor. Particularly, at the central portion of the spiral, a magnetic flux is generated which penetrates the plane of the conductive pattern. This magnetic flux enters the semiconductor substrate (non-insulating substrate). Now, when the magnetic flux is changed in accordance with the current flowing through the spiral conductive pattern an, eddy current is generated in the substrate by electromagnetic induction. Since this eddy current is generated in the direction opposing the magnetic flux, the magnetic flux density is reduced. As a result, self-inductance (L) of the spiral inductor is reduced and the quality factor (Q) is lowered. In a word, energy loss is large in the conventional spiral inductor. In addition, energy loss is large in an integrated circuit including such a spiral inductor as a whole since energy loss in the spiral inductor is large.
According to the conventional spiral inductor, in order to improve the Q value, two windings (two spiral inductors) are arranged opposed to each other (refer to Japanese Patent Laid-Open Publication No.2000-208704, for example). The two windings are connected in parallel. The winding directions of the two windings are symmetric and opposite to each other. According to the two windings, in the case where the current flows through the two windings, the mutual inductance can be reduced and the Q value can be high.
Furthermore, there is an inductance device in which a high Q value can be obtained by connecting two loops in parallel to reduce a whole resistance value (refer to Japanese Patent Laid-Open Publication No. 2002-508592, for example).
Still further, the high Q value can be obtained by removing four corners of a spiral coil wound around in a rectangular form (refer to Japanese Patent Laid-Open Publication No 3-89543, for example).
However, an inductor described in JP Laid-Open Publication No.2000-208704 is only applicable to the circuit employing the same two spiral inductors even though a high-frequency circuit uses only one spiral inductor instead of two in many cases.
In addition, the inductor described in JP Laid-Open Publication No.2000-208704 occupies a large area. Since the area of spiral inductor is thousands and ten thousands times as large as that of another device, if two spiral inductors are used, the area occupied by the spiral inductors is extremely increased.
Furthermore, it is not referred to at all in JP Laid-Open Publications No. 2000-208704, No. 2002-508592 and No. 3-89543 that the Q value is reduced because the magnetic flux induced by the current flowing in the inductor enters the semiconductor substrate.